The present invention relates to a polyurethane casting system and method.
Polyurethanes are polymers formed by combining a diol or polyol and an isocyanate. A diol is a dihydric alcohol having two hydroxyl (OH) groups, and a polyol is polyhydric alcohol having three or more hydroxyl groups. When combined, hydroxyl groups of the diol or polyol react with isocyanate (NCO) groups of the isocyanate to cause cross linking within the resulting polymer according to the general formula R1NCO+R2OHxe2x86x92R1NHCOOR2.
Polyurethane coatings are used in a wide variety of products to protect a less durable material, such as wood, from abrasion or weather damage. For such an application, it is desirable to produce a polyurethane coating that is abrasion resistant and weather resistant. Typically, such coatings are applied by brushing, spraying, or dipping processes.
U.S. Pat. No. 5,344,490 to Roosen et al. discloses a plasticized gypsum composition in the form of a polyurethane that may be used to coat wood. Bulk gypsum typically is composed primarily of dihydrous gypsum (CaSO4.2H2O), with smaller amounts of anhydrous gypsum (CaSO4), hemihydrous gypsum (2CaSO4.H2O), and hexhydrous gypsum (CaSO4.6H2O) contained therein.
Use of gypsum as a filler in polyurethane coatings causes several problems. The amount of water contained within a given quantity of bulk gypsum is difficult to predict because the amounts of anhydrous, hemihydrous, and hexhydrous gypsum will vary. At high temperatures during curing, some of the water present in the gypsum may separate from the gypsum, thus freeing the water to react with isocyanate in the polyurethane mixture to form a polyurea and carbon dioxide. Carbon dioxide produced during the formation of a polyurethane acts as a blowing agent, and causes the polyurethane to foam. Because the amount of water in gypsum is difficult to predict, the density of the resultant polyurethane foam also is difficult to predict and control.
To control water content, gypsum may be refined by baking out the water to produce anhydrous gypsum. However, baking consumes valuable time and energy. Likewise, refined anhydrous gypsum may be purchased from suppliers, but it is much more costly than bulk gypsum. In addition, anhydrous gypsum requires careful handling during shipping and manufacturing, so as not to expose it to water in the atmosphere or elsewhere, which it will readily absorb. These handling procedures are costly, and further make use of anhydrous gypsum expensive.
Another problem is that gypsum tends to soak up water present in the polyurethane reactants, further making it difficult to predict the amount of carbon dioxide that will be available to act as a blowing agent in the polyurethane reaction. For example, water added to produce additional foaming in a polyurethane reaction, or water present in castor oil or another polyol, may be absorbed by the gypsum.
In addition, the polyurethane reaction is exothermic, and heat from the reaction may cause water to separate from the gypsum, with less heat being required to separate water molecules from polyhydrous gypsum compounds than from hemihydrous or dihydrous compounds. As described above, water released from the gypsum will adversely and unpredictably affect foaming of the polyurethane. Thus, when using gypsum, the cure temperature of the polyurethane must be kept low to avoid release of water in the gypsum, thereby extending the total cure time and cycle time per polyurethane part.
Some water will remain attached to the gypsum through the curing process. This water later may separate from the gypsum when the cured polyurethane is exposed to high temperatures, as may occur upon direct exposure to sunlight. This may leave a cleaved and cratered surface on the polyurethane, making gypsum unacceptable for outdoor applications such as a south-facing wall.
Another problem with the use of gypsum is that it forms fluid mixtures that exhibit poor flowing characteristics when combined with other polyurethane constituents such as ethylene glycol and glycerin. The resulting gypsum mixture does not easily flow under the influence of gravity, and is therefore difficult to move from location to location during the polyurethane manufacturing process.
Forming a polyurethane coating on a substrate traditionally is accomplished by brushing, spraying, or dipping processes. None of these processes is capable of forming a polyurethane coating in a complex, three-dimensional shape, with features such as radii, surface texture, etc.
According to a preferred embodiment of the present invention, a mixture for forming a polyurethane is provided. The mixture typically includes a slurry including castor oil and calcium carbonate. Typically, the calcium carbonate is present in the slurry in a range of between about 40% and 70% by weight and the castor oil is present in the slurry in a range of between about 20% and 60% by weight. The mixture also includes an isocyanate mixed into the slurry such that isocyanate forms between about 10% and 50% by weight of the resulting mixture, the remainder of the resulting mixture being slurry. The mixture also may include a chain extender, cross-linking agent, or polymerization catalyst.
According to another preferred embodiment of the present invention, a polyurethane mixture is provided, including a slurry that includes about 33% castor oil, about 61% calcium carbonate, about 2% ethylene glycol, about 2% glycerin, about 1% to 2% titanium dioxide, about 0.1% desiccant, about 0.1% to 0.2% organotin, and an isocyanate mixed with the slurry such that the isocyanate forms about 20% to 25% of the resulting mixture, the slurry being the remainder of the resulting mixture.
According to another preferred embodiment of the present invention, a method of forming a polyurethane is provided. The method typically includes mixing castor oil and calcium carbonate to form a slurry, such that the calcium carbonate forms between about 40% to 70% of the slurry and the castor oil forms between about 20% and 60% of the slurry, adding an isocyanate to the slurry such that the isocyanate forms between about 10% and 50% of a resulting mixture, the remainder of the mixture being slurry, and mixing the isocyanate and the slurry, such that the isocyanate and slurry polymerize and form the polyurethane. The method also may include mixing a chain extender, cross-linking agent, or polymerization catalyst into the slurry.
According to another preferred embodiment of the present invention, a polyurethane casting system is provided. The polyurethane casting system includes a belt mold and a dispenser configured to deliver a polyurethane mixture to the belt mold. The polyurethane casting system also includes a loading mechanism configured to place a substrate onto the belt mold adjacent the polyurethane mixture. Typically, the polyurethane mixture is configured to bond to the substrate to form an integral article, and the integral article is configured to be removed from the belt mold as a unit. The casting system may also include a guide assembly for holding the substrate adjacent the polyurethane mixture on the belt mold, and a curing region for at least partially curing the polyurethane mixture.
According to another preferred embodiment of the present invention, a method of manufacturing an article is provided. The method includes mixing a polyurethane composition and dispensing the polyurethane composition onto a belt mold. The method also includes placing a substrate onto the belt mold adjacent the polyurethane composition, and at least partially curing the polyurethane composition on the belt mold to adhere the polyurethane composition to the substrate and form an integral article.
According to another preferred embodiment of the present invention, a manufactured article is provided. The manufactured article typically includes a substrate and an at least partially cured polyurethane composition positioned adjacent the substrate, where the partially cured polyurethane composition is formed from mixture including about 10% to 50% isocyanate and about 50% to 90% slurry. The slurry typically includes about 40% to 70% calcium carbonate and about 20% to 60% castor oil.
According to another preferred embodiment of the present invention, a polyurethane product is provided, the polyurethane product being formed by a process including mixing an alcohol having at least two hydroxyl groups and calcium carbonate to form a slurry, such that the calcium carbonate forms between about 40% and 70% of the slurry, adding an isocyanate to the slurry to form a resulting mixture, the isocyanate being added in an amount effective to form a polyurethane, dispensing the resulting mixture into a belt mold, positioning a substrate adjacent the resulting mixture on the belt mold, and at least partially curing the resulting mixture on the belt mold to adhere the resulting mixture to the substrate.